error: add error_setg()

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

error.h

@@ -29,6 +29,12 @@ typedef struct Error Error;
  */
 void error_set(Error **err, ErrorClass err_class, const char *fmt, ...) GCC_FMT_ATTR(3, 4);
 
+/**
+ * Same as error_set(), but sets a generic error
+ */
+#define error_setg(err, fmt, ...) \
+    error_set(err, ERROR_CLASS_GENERIC_ERROR, fmt, ## __VA_ARGS__)
+
 /**
  * Returns true if an indirect pointer to an error is pointing to a valid
  * error object.